1. Field of the Invention
The present invention is directed to optical lensing and more particularly to a method and apparatus for lensing a beam of light using a gas.
2. Description of the Prior Art
Special resonator problems exist with high-power lasers in which energy flux must be reduced to intensity levels tolerable by the resonator mirrors and other optical components. If the resonator relies solely on diffraction spreading of the beam for intensity reduction, very long spacing is required and alignment is problematic. One option for high-power resonators is to produce beam divergence by lensing with gases, i.e. using density or species gradients to produce lens effects. Unlike solid-state lenses, gases can provide a low-absorption, flowing lens structure conducive to low thermal distortion and immune from permanent damage.
The bending of light rays basically requires establishing a refractive-index gradient transverse to the ray path. For example, in the bending of light by a glass prism in air, the prism faces slant across the rays and the sharp transition from the refractive-index of glass to that of air provides the requisite transverse refractive-index gradient. Obtaining divergent or convergent lasing of a light beam additionally requires establishing a gradual variation in the transverse refractive index gradient so that the rays composing the beam experience different amounts of bending. For example, conventional glass-in-air lenses have curved surfaces so that the angle of the surface to the rays and hence the amount of bending varies across the beam.
The index of refraction of a gas largely depends on its molecular density and on the molecular species constituting the gas. Lensing by means of gases therefore requires gradients in either the molecular density or the molecular species. The density in turn depends on the pressure and temperature of the gas. It is noted that as used herein and as well known in the art, the term "lensing" refers to either converging or diverging the light beam. Lensing by means of pressure gradients within a gas flow is, for example, disclosed in U.S. Pat. No. 4,582,398 issued to T. G. Roberts and lensing by means of temperature gradients is disclosed in U.S. Pat. No. 3,435,363 issued to C. K. N. Patel.
These and other related approaches are unsuitable for laser applications because the required lensing strength cannot be conveniently produced and because their weak lensing gradients are easily disrupted by the laser energy, which in turn degrades the quality of the laser beam.